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Grain Silos and Dust Explosions

April 30, 2018

Fires and explosions can be unexpected, but for either to happen, an uninhibited chemical reaction is required. In other words, an uninterrupted sequence of events must take place. To determine how these unexpected fires and explosions occur, fire investigators must often look past expected scenarios and explore less common sources.

A fire or explosion requires three elements for ignition: heat, fuel, and oxygen (or an oxidizing agent). When investigating the cause and origin of fires and explosions, one or more of these elements can be hidden from sight – or come from an unexpected source. For example, fires can start inside grain silos with no obvious heat source, burning up tonnes of material stored inside. Explosions can occur in industrial factories and workshops without a clear source of fuel or ignition.

In the case of self-combusting silos, the ignition source is the result of a biological process. After being harvested, plant material called silage is typically stored in silos for livestock use. The silage is naturally preserved in the silo through a controlled fermentation process. A well-designed silo with properly prepared and distributed silage will allow the silage to ferment until it reaches a stable state void of oxygen. However, if oxygen becomes readily available to the silage, the fermentation process can occur too rapidly, allowing excessive bacterial growth through a process that produces heat. Pockets of heated material can form within the silo, and under the right conditions these pockets can reach temperatures high enough for spontaneous combustion! Once ignited, a silo with dry silage and a constant supply of oxygen can go up in flames, taking thousands of dollars of material with it. To prevent these situations the amount of oxygen in the silo must be limited, which naturally limits the growth of the heat-producing bacteria.

Dust explosions are an event in which the fuel component can be easily overlooked. Often a hazard in grain elevators, wood manufacturing, and even industrial plants processing organic materials (such as flour, cocoa and peat moss), fine particles created as a product or byproduct must be controlled. A high concentration of fine particles in the air and collected on equipment can act as a fuel source, and with a ready supply of oxygen already thoroughly mixed, all that is required is a spark or source of heat. This can come from tools and equipment being used in the area, or from something as innocuous as static electricity – a spark of energy travelling through the air. Proper ventilation is an important preventative measure in rooms where combustible airborne particles are produced, and other measures such as grounding equipment should also be taken where this is a concern.

While some fires and explosions have obvious heat and fuel sources, others require digging deeper into the chemistry of fires and the natural processes of the world around us in order to identify these sources. It is the challenges of situations like these that keep our fire and explosion investigators diligent and excited about each new file.

Written by Brandon Cathcart


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